Oil Question

Rod, two 20W-50s can have slightly different pressures as yours do because the viscosities have to fall in a range. They can be on spec without being exactly the same.

Avoid using additives. There's no way to know how they will work with the oil's chemistry.

Mobil 1 15W-50 is a quality full synthetic with a good amount of ZDDP blended in. It's also reasonably priced. Maybe try it next time. Good luck.